Various types of antenna devices exist, including dielectric resonator antennas (DRAs), high dielectric antennas (HDAs), dielectrically loaded antennas (DLAs), dielectrically excited antennas (DEAs) and traditional conductive antennas made out of electrically conductive materials.
DRAs are well known in the prior art, and generally are formed as a pellet of a high permittivity dielectric material, such as a ceramic material, that is excited by a direct microstrip feed, by an aperture or slot feed or by a probe inserted into the dielectric material. A DRA generally requires a conductive groundplane or grounded substrate. In a DRA, the main radiator is the dielectric pellet, radiation being generated by displacement currents induced in the dielectric material.
HDAs are similar to DRAs, but instead of having a full ground plane located under the dielectric pellet, HDAs have a smaller ground plane or no ground plane at all. DRAs generally have a deep, well-defined resonant frequency, whereas HDAs tend to have a less well-defined response, but operate over a wider range of frequencies. Again, the primary radiator in the dielectric pellet.
A DLA generally has the form of an electrically conductive element that is contacted by a dielectric element, for example a ceramic element of suitable shape. The primary radiator in a DLA is the electrically conductive element, but its radiating properties are modified by the dielectric element so as to allow a DLA to have smaller dimensions than a traditional conductive antenna with the same performance.
A further type of antenna recently developed by the present applicant is the dielectrically excited antenna (DEA). A DEA comprises a DRA, HDA or DLA used in conjunction with a conductive antenna, for example a planar inverted-L antenna (PILA) or planar inverted-F antenna (PIFA). In a DEA, the dielectric antenna component (i.e. the DRA, HDA or DLA) is driven, and a conductive antenna located in close proximity to the dielectric antenna is parasitically excited by the dielectric antenna, often radiating at a different frequency so as to provide dual or multi band operation. Alternatively, the conductive antenna may be driven so as parasitically to drive the dielectric antenna.
An important problem facing antenna designers, in particular today where many portable appliances such as computers, mobile telephones, computer peripherals and the like communicate with each other in a wireless manner, is to provide good diversity within a small space. In telecommunications and radar applications it is often desirable to have two or more antennas that give a different or diverse ‘view’ of an incoming signal. Generally speaking, the different views of the signal can be combined to achieve some optimum or at least improved performance such as maximum or at least improved signal to noise ratio, minimum or at least reduced interference maximum or at least improved carrier to interference ratio, and so forth. Signal diversity using several antennas can be achieved by separating the antennas (spatial diversity), by pointing the antennas in different directions (pattern or directional diversity) or by using different polarisations (polarisation diversity). Antenna diversity is also important fox overcoming the multi-path problem, where an incoming signal is reflected off buildings and other structures resulting in a plurality of differently phased components of the same signal.
A significant problem arises when diversity is required from a small space or volume such that the antennas have to be closely spaced. An example of this is when a PCMCIA card, inserted into a laptop computer, is used to connect to the external world by radio. Most high data rate radio links require diversity to obtain the necessary level of performance, but the space available on a PCMCIA card is generally of the order of about ⅓ of a wavelength. At such a close spacing, most antennas will couple closely together and will therefore tend to behave like a single antenna. In addition, there is little isolation between the antennas and, consequently, there is little diversity or difference in performance between the antennas. As a rule, about −20 dB coupling (isolation) is the target specification between antennas operating on the same band for a PCMCIA card. For access points (in WLAN and the like applications), which are rather like micro-base stations, even greater isolation is required, about −40 dB being desirable. Such high isolation is extremely hard to achieve with conventional antennas when the access points are the size of domestic smoke alarms and less than a wavelength across. Similarly with laptop computers, isolation between WLAN and Bluetooth® antennas of −40 dB or more is seen as desirable.
A method of creating good diversity at the Wireless Local Area Network (WLAN) frequency of 2.4 GHz has been published [“Printed diversity monopole antenna for WLAN operation”, T-Y Wu, et. al., Electronics Letters, 38, 25, Dec. 2002]. This paper describes how to remove the ground plane on the underside of a printed circuit board (PCB) so that the end section of a microstrip on the top surface becomes a radiating monopoly. This is shown in FIG. 1 of the present application Wu et al., also describe how a T-shaped section of ground plane between the two antennas can help to increase port isolation between them. Further details are presented in [“Planar Antennas for WLAN Applications”, K-L Wong, National Sun Yat-Sen University, Taiwan, presented at the 2002 Ansoft Workshop and available on the Ansoft website].
The antenna system discussed above is relatively narrow band and no method of extending the bandwidth or other aspects of antenna performance, is offered. As described in the paper by Wu et al, this type of antenna does not have sufficient bandwidth to be used in a mobile communications system.
It is part of accepted antenna theory that ‘fat’ monopoles can be designed to have wider band performance than ‘thin’ monopoles, see for example, [“The handbook of antenna design”, O. Rudge, et al., Peter Peregrinus Ltd, 1986] where rectangular and conical shaped monopoles are shown to have very broadband responses. A recent paper [“Annular planar monopole antennas”, Z. N. Chen, et. al., lEE Proc-Microw. Antennas Propag., 149, 4, 200–203, 2002] describes how a monopole shaped as a circular disk or annulus can have broadband impedance and radiation characteristics A recent book [“Broadband microstrip antennas”, G. Kumar & K. P. Ray, Artech House, 2003] describes how the fat dipole concepts can be extended to printed microstrip antennas (MSAs). FIG. 2 shows the general design of an MSA and Kumar & Ray show that rectangular, triangular, hexagonal and circular printed microstrip antennas all have broadband properties. U.S. Pat. No. 5,828,346 discloses a diversity card antenna with a pair of monopole antennas mounted in two corners of a printed circuit board (PCB) substrate. The monopoles are formed respectively as F and inverse F antennas, L and inverse L antennas, or F and L antennas so as to provide pattern diversity. The antennas are alternately fed by a switching device so as to eliminate fading.
EP 0 720 252 (AT&T) discloses a multi-branch patch antenna in which four conductive patches are mounted on a dielectric substrate which is itself mounted on a conductive groundplane. A conductive “septum” forms a cross on the sur face of the dielectric substrate, separating the patch antennas, and contacts the groundplane. The patch antennas are located above the groundplane.
All of the references identified above are hereby incorporated into the present application by way of reference, and are thus to be considered as part of the present disclosure.